Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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m e present invention relates to gas turbine engines,
and more particularly to a compressor impeller shroud construc-
tion for a gas turbine engine.
The compressor impeller in a Pratt & Whitney PT6
engine is the last compressor stage, being downstream of a
series of alternating axially arranged compressor rotors and
stators. It has an axial conponent and a radial component with
~he outlet portion of the impeller discharging the air so
compressed radially into a tàngential annular diffuser. m e
impeller hub is mounted to the compressor shaft while the
impeller blades are unshrouded and are integral at their respec-
tive bases with the hub. The hub which supports the blades has
a portion which is almost cylindrical and is massive at its
inlet end and spreads out to a much thinner radial plate at
its outlet end.
A static impeller shroud is conventionally mounted to
the case of the engine in the compressor section by means of a
support ring fixed to the case.
In the PT6 engine, struts extend across the gas path
at the air intake supporting the outer case against the inner
case which supports a thrust bearing. m e thrust bearing is
the support for the shaft and thus all the rotating parts of
the compressor including the impeller. In other words, the
only common datum for the tips of the impeller blades and the
impeller shroud, which must be kept at close tolerances
thereto, is the thrust bearing. m us, thermal expansion grows
from the thrust bearing for both the impeller blade tips and
the impeller shroud. However, the path of expansion growth
for the impeller shroud is much greater than for the impeller
tips. m e expansion growth for the impeller is directly
through the rotor assembly to the thrust bearing, while the
expansion growth for the impeller shroud is through the outer
case.
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Furthermore, it has been found that the outer peri-
phery of the impeller can be distorted as a result of centri-
fugal forces due to its high rotational speed. Since the impel-
ler shroud is fixed to`the diffuser, it will remain static
while the tips of the impeller blades will tend to reduce the
clearance between the shroud and the blade tips. Furthermore,
in a typical PT6 engine, the diffuser is fixed to a diaphragm
structure which extends from the casing through the diffuser to
a bearing on the gas generator side of the engine. During
surge conditions,-the pressure differential across this
diaphragm structure causes axial deflection of the impeller
shroud relative to the impellèr blades in a direction opposite
from the mechanical deflection of the impeller blades due to
centrifugal forces.
It is an aim of the present invention to provide an
improved impeller shroud construction, and in particular, the
support of such impeller shroud.
It is a further aim of the present invention to provide
an impeller shroud which will deflect in the same manner as the
impeller outlet portion so as to maintain a relatively constant
clearance.
It is a rurther aim of the present invention to provi-
de a more direct thermal expansion growth path and particularly
one that is of similar distance from its common datum with the
impeller such that the thermal expansion or contraction of the
impeller shroud will be similar to that of the impeller so as
to maintain relatively constant blade tip clearance.
A construction in accordance with the present invention
comprises a compressor for a gas turbine engine comprising an
engine housing a rotor assembly, including an impeller. lhe
impeller has an inlet portion having an axial component and
an outlet portion having a radial component. The impellerincludes
1~1()34
a hub having a flared cantilever radial configuration and a
plurality of blades thereon, and the blades have unshrouded
tips. The rotor assembly is mounted to the engine housing by
bearing means. A stator assemb]y mounted to the housing
includes a shroud surrounding and concentric with the rotor
assembly. The shroud includes an impeller shroud member.
The impeller shroud has a flared shape with an inlet portion
fixed to the shrou~ and the outlet portion being unsupported.
m e impeller shroud surrounds, with minimum clearance, the
blade tips of the impeller, whereby any deflection of the
impeller shroud will occur at the outlet portion thereof in
correspondence with any deflection of the outlet portion of
the impeller.
m us, the thermal expansion growth distance of the
impeller shroud is substantialiy similar to the e~pansion
growth distance of the impeller from the bearing means. The
fact that the impeller shroud is fixedly connected to the
remainder of the shroud at its inlet portion, that is, at the
area of smallest diameter of the impeller shroud, the remainder
of the impeller shroud is thus cantilevered, and any deflection
caused by pressure differences acting on the impeller and
impeller shroud will cause the cantilevered outlet portions
thereof to deflect in unison, thereby maintaining minimum
clearance between the shroud and the blade tips.
In a more specific embodiment of the present invention,
the outlet portion of the impeller shroud is provided with
sealing means adapted to sealingly engage a partition member.
m e sealing means may be in the form of sealing rings mounted
on the periphery of the outlet portion of the impeller shroud
adapted to slidingly engage the partition member.
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Having thus generally described the nature of the
invention, reference will now be made to the accompanying
drawings, showing by way of illustration, a preferred embodiment
thereof, and in which:
Figure 1 is an axial cross-section of the
compressor section of a gas turbine
engine incorporating the present
invention, and
Figure 2 is an enlarged fragmentary axial
lQ cross-section of a detail shown in
: Figure 1.
Referring now to the drawings, there is shown in a
compressor section 10 of a gas turbine engine an outer case
12 which defines an annular inlet 14. The annular case 12
surrounds the compressor section 10, and the struts 16 are
spaced apart peripherally across the annular inlet 14. A
thrust bearing 18 surrounds the shaft and mounts the rotor
assembly 20. m e thrust bearing 18 is mounted within the inner
case 24 which is connected directly to the struts 16 and is
thus supported by the outer case 12. m e outer case 12 also
mounts a stator assembly 22 which comprises shroud members 26
for the rotor assembly which will be described further.
Annular inlet 14 permits an air flow to enter the
compressor section 10 of the engine, and the path of the air
flow is axial, past the rotor and stator assemblies to finally
reach the impeller 28 which turns the air flow from an axial
.path to a radial path through the diffuser 30 and then axially
through the pipes 32 into-the gas generator section.
The rotor assembly includes a somewhat cylindrical
rotor support member 34 mounting rotor blade hubs 36 by means
of the bolts 38. Rotor blades 40 represent the first stage of
rotors in the compressor section. A further cylindrical rotor
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support member 45 is bolted to the hub 36 by means of bolts 38
and to the hub 46 of the impeller 78 by means of the bolts 47.
~he cylindrical rotor support member 45 mounts the second and
third stages of rotor blades, identified 42 and 44. These
blades are unshrouded and are adapted to rotate with close
tolerances between the rotor blade tips and the static shroud
26 forming part of the stator assembly 22.
m e impeller 28 includes a hub 46 mounted on the shaft
(not shown~ and mounts the radially extending impeller blades
52 having blade tips 54. The impeller blades have an axial
component in the inlet portion 48 of the impeller and are
turned such that the blades have a radial component at the
outlet 50 of the impeller. m e diffuser 30 is mounted to the
support ring 102 of the case 12 and supports, by means of the
diaphragm 56, the roller bearing which supports the rotor
assembly by means of a support shaft. m e impeller 28, on the
other hand, is in fact cantilevered in that the outlet section
50 of the impeller is somewhat-flared and is, of course,
unsupported at its extremities or, that is, in the outlet
0 segment.
m e stator assembly includes the intake outer case
wall 62 which is mounted directly to the struts 16 and is
integral with outer case wall 12. m e shroud 26 is provided
with a flange 68 and is adapted to be bolted to the intake
outer case wall 62 by means of bolts 70. Shroud 26 acts as a
circumferential shroud for the rotor blades 40 of the first
stage and mounts the stator vanes 72 of the first stator stage.
m e shroud 26 is in ring sections and is bolted at flanges and
bolt assembly 74 and also mounts a shroud ring for the second
rotor stage blades 42. Shroud 26 also mounts the stator blades
76 of the second stage, and finally the stator vanes 78 of the
third stage.
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The stator assembly shroud 26 has a configuration
of an axial cylinder connected by bolts 70 to the inlet outer
case wall 62 at one end. A flared impeller shroud 82 is
mounted to the flange 80 on the shroud 26 by means of bolts
86. As shown in Figure 2, shims 84 may be provided between
the flange 80 and the bosses 88 provided on the impeller
shroud 82 for the purpose of being bolted by means of the bolt
86. The provision of the shims 84 allows for axial displace-
ment or adjustment of the impeller shroud relative to the
impeller. Shims can be removed without taking apart the
assembly. Between each boss 88 are slots, which provide for
bleeding air from the compressor.
The impeller shroud 82 is flared and follows a
similar curvature to that of the blade tips 54 of the impeller
28. The impeller shroud 82 has an outlet portion 92 near the
outlet portion 50 of the impeller 28. Mounted to the impeller
shroud at the outlet portion 92 is an axially extending
cylindrical flange member 94 provided with a peripheral channèl
96 in which are located a pair of sealing rings 98. The
sealing rings 98 are adapted to be in contact with the adaptor
ring 100 provided on the support ring 102 mounted to the casing.
There is no fixed connection between flange 94 and the sealing
rings 98 with the adaptor ring 100~ Rather, the sealing rings
98 allow relative movement of the impeller shroud outlet
portion 92 relative to the adaptor ring 98 while maintaining a
sealing contact therewith.
It can be seen from the above description and the
drawings that the thermal expansion growth path of the stator
assembly mounting the impeller shroud 82 is of a similar length
through the shroud 26, the case 62, strut 16, and inner case
24 to the thrust bearing 18, as the thermal expansion growth
path of the impeller 28 through the rotor assembly 20, the hub
36, rotor support 34, and thus to the thrust bearing 18.
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The rotor support member 45 and the shroud 26 are the contact
surfaces of the gas path of the compressor, and thus the
thermal characteristics of both surfaces will be the same.
m erefore, the expansion growth due to thermal considerations
of the impeller shroud 82 will be similar to that of the
impeller 28.
In operation, there may be considerable pressure
differences arising between the gas generator section and the
compressor section. For instance, under surge conditions,
even though the impeller might deflect axially in the direction
of the thrust bearing due to centrifugal forces, and the
diffuser might be forced to move axially slightly in the
opposite direction because of the diaphragm support structure,
the impeller shroud, which is cantilevered from its smallest
diameter and which mounts the sealing rings 98 against the
diffuser-structure, is allowed to move relative to the shroud
and deflect in the same direction as t~e slight deflection of
the impeller towards the thrust bearing.
This of course, is in comparison with the conventio-
nal manner of mounting the impeller shroud from the outercasing which would not allow for such deflection :in the outlet
portion of the impeller shroud.
